The toughness of epoxy-poly(butylene terephthalate) blends

Blends containing 5% poly(butylene terephthalate) (PBT) in an anhydride-cured epoxy with three different PBT morphologies were studied. The three morphologies were a dispersion of spherulites, a structureless gel and a gel with spherulites. The average fracture toughnesses, K Ic , and fracture energies, G Ic , for those morphologies were 0.83, 2.3 and 1.8 MPa m 1/2 and 240, 2000 and 1150 J m −2 , respectively. These values should be compared with the values of 0.72 MPa m 1/2 and 180 J m −2 , respectively, for the cured epoxy without PBT. The elastic moduli and yield strengths in compression for all three blend morphologies remained essentially unchanged from those of the cured epoxy without PBT, namely, 2.9 GPa for the modulus and 115 MPa for the yield strength. The fracture surfaces of the cured spherulitic dispersion blends indicate the absorption of fracture energy by crack bifurcation induced by the spherulites. The fracture surfaces of the cured structureless gel blends indicate that fracture energy was absorbed by matrix and PBT plastic deformation and by spontaneous crack bifurcation. But phase transformation of the PBT and anelastic strain of the matrix below the fracture surfaces may account for most of the large fracture energy of the cured structureless gel blends.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44733/1/10853_2004_Article_BF00366876.pd

Monitoring of hydration processes in cement materials by broadband Time-Domain-Reflectometry Dielectric Spectroscopy

Prior work in our laboratory demonstrated a continuous monitoring of the chemical state of water in hydrating cement paste, over the frequency range 10 kHz to 8 GHz and from initial mixing to several weeks cure. The broadband complex permittivity is obtained over the range using Time-Domain-Reflectometry (TDR) Dielectric Spectroscopy, using Fourier transform methods and an embedded capacitance sensor. Three fundamental signals are identified, corresponding to unreacted free water appearing near 10 GHz, bound-water attaching to developing microstructure near 100 MHz, and grain polarization occurring around 1 MHz. The three signal components are fit to appropriate molecular models as a function of cure time and monitored throughout the process. The result is 1) a free-water relaxation which monitors the disappearance of water into hydration and thus follows percent hydration, and 2) a bound-water relaxation which monitors water attaching to developing microstructure and thus monitors formation of this microstructure, and 3) a grain-polarization component which monitors developing microstructure. Our current work focuses on investigating changes in this relaxation spectrum and its cure evolution with changes in chemistry and processing conditions. Using the full frequency transform, we explore relative changes in various signal components with changes in ionic strength and backfilling of the pore space with excess free water. Monitoring the transient amplitude directly, we follow specific frequency components by monitoring corresponding delay times and follow reaction rates in the time domain. Changes in reaction rate are compared for changes in temperature, addition of accelerants and retarders, and other factors. Reaction rates obtained by TDR are also compared with rates obtained by other measures of cement hydration; such as heat evolution by isothermal calorimetry, bound-water formation by thermogravimetric analysis, and calcium hydroxide production by differential scanning calorimetry. The transient analysis can be integrated with a small portable TDR sampler to form a robust cure-monitoring system usable in the field. Copyright © 2007 MS&T\u2707®

Monitoring of cement hydration by broadband time-domain-reflectometry dielectric spectroscopy

The broadband complex permittivity is monitored continuously in hydrating cement paste over the frequency range of 10 kHz-8 GHz and from initial mixing to several weeks of cure. Measurements are made by time domain reflectometry (TDR) dielectric spectroscopy, using an adjustable capacitance sensor, which can be embedded in the material in situ. The results are fit to a relaxation model, which includes terms representing (1) a Cole-Davidson relaxation near 1 MHz, which grows initially and then decreases with an advancing cure; (2) a Debye relaxation near 100 MHz, which grows initially and then decreases with an advancing cure; (3) a free-water relaxation near 10 GHz, which decreases with an advancing cure; and (4) an ion conductivity and electrode polarization, which decreases with an advancing cure. The model is fit continuously as function of cure time extracting parameters for the relaxation amplitudes, relaxation frequencies, and distribution parameters as a function of cure time. The results are contrasted with measurements in tricalcium silicate and tricalcium silicate with varying ion content, revealing differences that may indicate the nature of the processes involved. Alternative methods for extracting reaction-rate information directly from the TDR transient are presented, providing a robust monitoring procedure usable in the field. Such methods are demonstrated using a variation in temperature and comparison with measurements of reaction heat obtained by calorimetry. © 2004 American Institute of Physics

Smith-chart diagnostics for multi-GHz time-domain-reflectometry dielectric spectroscopy

A time-domain-reflectometry Smith-chart display is demonstrated to be a valuable diagnostic tool in a variety of situations in time-domain-reflectometry dielectric spectroscopy. A relative reflection coefficient is formed by dividing the Laplace transform of the reflected sample transient by the Laplace transform of the empty-sensor transient and displaying in the complex plane, with the approximate sensor admittance read from susceptance and conductance circles. The Smith chart provides, as a diagnostic tool, an initial estimate of the dielectric behavior in the multi-GHz range and a means of identifying artifacts in acquisition and Laplace transform, in a way which does not require multiple steps of calibration and is only one step removed from the direct transient. Results are presented for a simple 3.5-mm flat sensor immersed in various liquid media, showing variations in the Smith chart for typical variations in sample permittivity, loss, and conductivity. Results are matched to vector network analyzer (VNA) measurement over an identical frequency range, as well as to finite-element field simulation. Results are also presented for a 3.5-mm sensor with various terminating pin lengths, typically employed at low frequencies and low permittivity media to increase sensor capacitance. For an unshielded pin, the Smith chart detects reflections from sample boundaries and measures the effectiveness of shielding used to eliminate these reflections. For a shielded pin, it characterizes the effect of pin length on the susceptance variation and the onset of pin resonance at high frequencies and high-permittivity values. The effect of artifacts appearing in the Smith chart on the actual calibration is shown by tracking them through the calibration process to the final result. Results are also presented for a 9-mm flat termination used for concrete hydration monitoring, showing effects of transmission-line discontinuities within a terminating plug and the onset of waveguide-like modes in a surrounding shield, with results compared to VNA measurement. © 2012 American Institute of Physics

A time-domain reflectometry method for automated measurement of crack propagation in composites during mode i DCB testing

This article describes a new technique for automated measurement of crack initiation, growth, and propagation in composite materials during mode I double cantilever beam (DCB) testing. The proposed method uses time-domain reflectometry (TDR) to detect changes in geometry and electromagnetic properties (dielectric or magnetic) along a transmission line that can be embedded in or bonded to the surface of the specimen. Two types of transmission line TDR sensors are evaluated (IM7 carbon fiber and ARACON) during DCB tests. A P-SPICE transmission-line simulation model is used to verify the baseline signal response for the DCB sensor and the sensitivity for crack detection, with good agreement. Comparison with standard visual methods in DCB testing showed excellent correlation in crack location, crack propagation (LC), and the interlaminar fracture toughness (GIC) values. The TDR sensor design and model-based parametric studies are carried out to determine optimal sensor geometry and configuration. The results demonstrate that the TDR-based method can measure crack propagation parameters at high resolution and accuracy, in an automated manner using low-cost sensors. © 2006 SAGE Publication